This invention relates to solid fuel heating appliances, and more particularly to appliances which burn wood, densified or compacted wood products, coal, charcoal, peat, compacted trash or similar solid fuels and utilize a combustor to promote oxidation of oxidizable species in the exhaust of such appliances.
U.S. patent application Ser. No. 173,155 filed July 28, 1980 (assigned to the assignee of this application) now abandoned, discloses the use of a combustor in a solid fuel heating appliance. The solid fuel heating appliance in the form of a wood burning stove uses a catalytic converter as a combustor which serves to provide more complete burning or oxidation of the volatile and particulate organic substances present in gases rising from burning wood in a wood burning stove and especially those solid particles and resinous and oily droplets that cause the dense smoke which upon deposition on the inside surface of the flue pipe or chimney are generally known as creosote. More particularly, a catalytic converter which comprises noble-metal catalysts on a suitable substrate reduces the ignition temperatures of carbon monoxide and the lower boiling, more volatile hydrocarbons present in the exhaust issuing from the combustion of wood. As the hydrocarbons and carbon monoxide burn, the temperature of the catalyst and its substrate is raised which increases its catalytic activity. The elevated temperature pyrolyzes and cracks the higher molecular weight hydrocarbons occurring in the smoke as solid particles and oily droplets, converting them to volatile compounds which readily mix with oxygen present and thereby leading to their rapid oxidation. Temperature continues to rise until the system reaches a temperature at which there is equilibrium between the inlet gas temperature, flow rate and the amount of oxidizable material. This temperature is typically 600.degree. C. to 900.degree. C. for a properly sized catalyst system. At these temperatures, oxidation proceeds very rapidly to completion if the catalytic converter has the appropriate volume and internal surface area. As converter temperatures increase, the exhaust gas temperature rises above the ignition point of an increasing number of its constituents so that the catalytic combustion process is augmented by thermal combustion. This high temperature also breaks the complex hydrocarbons and other combustibles (including solid particulates entrained in the combustion gases) into compounds which will burn more easily.
In order to optimize the performance of the combustor or catalytic converter, the combustor should be positioned within or very near the fire box of the wood burning stove. This proximity assures more consistant light off in burning by precluding any substantial cooling of the exhaust gases prior to entry into the combustor. However, positioning of a combustor within or very near the fire box results in the direct impingement on the combustor by flames within the fire box. This flame contact with the combustor can result in severe thermal shock of the combustor, and it has been found that this shock does result in the degradation of the catalytic converter. More specifically, the catalytic wash coat applied to the combustor will flake under the conditions of direct flame impingement. This in turn can result in the blockage of cells within the combustor and the loss of catalysis which can produce a shorter combustor life and a reduced combustor performance.